Parking brakes are used whenever non-mechanical components are used in a powertrain, especially in automatic transmissions with hydrodynamic torque converters.
Motor vehicles with automatic transmission are thus usually equipped with a parking brake in order to prevent the vehicle from rolling unintendedly away when the vehicle is stopped and the engine is turned off. The parking brake now blocks the transmission power take-off shaft connected to the driving wheels in such a way that they rotate in unison as a consequence of an actuating signal initiated by the driver, for example, by a pawl arranged in the automatic transmission being caused to mesh with the teeth of the parking brake wheel arranged on the transmission power take-off shaft.
Many different mechanical, electrohydraulic as well as electromechanical systems are known as transmission means between such a parking brake, designed, for example, as a pawl, and the actuating device of the parking brake, which said actuating device is arranged in the passenger compartment of the motor vehicle. Mechanical transmission means for actuating the parking brake comprise, for example, a Bowden cable between the selector lever arranged in the passenger compartment and the transmission control arranged at the automatic transmission.
To improve the possibilities of design embodiment of the free arrangement of the actuating device in the passenger compartment and nevertheless to also guarantee an acoustic uncoupling of the actuating device from the powertrain of the motor vehicle, there has increasingly been a shift in modern motor vehicles or automatic transmissions to the shift-by-wire principle, especially because modern automatic transmissions are also increasingly controlled fully electronically.
There usually is no mechanical connection whatsoever any more between the actuating element in the passenger compartment and the gearbox itself in such “shift-by-wire” transmissions. The shift command is rather transmitted from the actuating means or from the selector lever of the automatic transmission mostly exclusively by means of electrical or electronic signals.
Concerning the parking brake, this means, in other words, that the transmission means provided for actuating the parking brake between the selector lever and the adjusting means of the parking brake, by which means the motion of the pawl is brought about, is preferably also designed as a purely electrical or electronic signal connection.
However, independently from the actual design of the transmission means, it is required in all parking brake systems that unintended engaging of the parking brake, for example, as a consequence of failure of hydraulic or electrical components, be avoided during driving. The parking brake shall consequently be engaged with certainty only when the lever position P is actually engaged on the selector lever or when—as is present in some automatic transmissions—the P button was pressed or when—for example, after the driver has left the motor vehicle—the parking brake was possibly engaged automatically.
DE 43 22 523 A1 discloses an electrohydraulic control means for a motor vehicle automatic transmission with a functional connection designed as an electrical connection between the actuating means of the automatic transmission in the interior space of the vehicle and the electrohydraulic transmission control, in which the different gears of the automatic transmission are engaged by admitting pressure to electrohydraulic shifting members in the transmission control, whereas shift position P is engaged by depressurizing all shifting members. The arrangement described in this document consequently leads at first to the circumstance that the parking brake is engaged when pressure is eliminated not only in shift position P of the automatic transmission but also in all gears of the selector lever.
The parking brake is engaged here via a spring-type brake actuator, while the parking brake is disengaged hydraulically via the oil pressure circuit of the automatic transmission. To avoid unintended engagement of the parking brake when, e.g., there is a defect in the transmission oil supply when the selector lever is not engaged in shift position P, an additional control means is provided, which is controlled by the selector lever by means of a Bowden cable and prevents the parking brake from becoming engaged via the spring-type brake actuator until the selector lever is brought into shift position P.
DE 198 34 156 A1 describes a parking brake for a motor vehicle automatic transmission, in which the pawl of the parking brake is likewise engaged by means of a spring-type brake actuator and is released by hydraulic pressure from the oil pressure circuit of the automatic transmission, while the spring-type brake actuator is prestressed at the same time. To prevent the parking brake from engaging unintendedly when the hydraulic pressure is lost, a mechanical ball locking mechanism of the hydraulic parking brake actuation is provided, which is actuated via an electromagnet. As long as the electromagnet is energized, the ball locking mechanism is held in the locked position, so that the parking brake remains in the released position. If the selector lever is brought into shift position P, the electromagnet is switched off, as a result of which the ball locking mechanism is released, which in turn leads to the engagement of the parking brake by means of the prestress of the spring-type brake actuator.
DE 10 2004 030 006 A1 shows a parking brake device, in which the parking brake pawl is likewise engaged by means of a spring-type brake actuator and is again released by hydraulic pressure from the oil pressure circuit of the automatic transmission while the spring-type brake actuator is compressed at the same time. To avoid unintended engaging of the parking brake when the hydraulic pressure is possibly lost, a mechanical pawl holder is actuated by energizing an electromagnet. As long as the electromagnet is not energized, the pawl holder is held in the released position, so that the parking brake remains in the released position. If the electromagnet is energized, the pawl holder is released, and the parking brake is engaged because of the prestress of the spring-type brake actuator.
However, the devices known from the state of the art for the electromagnetic or mechanical securing of the parking brake pawl in the disengaged position for the case of loss of hydraulic pressure, especially the electromagnetically secured ball holders as well as the electromagnetically released pawl holders, are complicated and therefore cost-intensive in terms of design and production. These are demanding precision mechanical assembly units, which require a very precise coordination and dimensional accuracy of the components and are, moreover, sensitive to different ambient conditions, and such holding means therefore also need to be arranged in an especially protected manner.